In another hurdle for Royal Dutch Shell's Arctic drilling program, an ice-handling vessel playing a key role in the operation has returned to Dutch Harbor after a gash was discovered in its hull.

The Fennica, a 380-foot Finnish vessel, was damaged Friday as it headed for the drilling grounds in the Chukchi Sea off the northwest Alaska coast, with a state-certified marine harbor pilot on board handling it. The vessel is one of 29 Shell plans to send to the area this summer.

In addition to duties that include scouting for wayward chunks of sea ice and preventing them from threatening other vessels, one of the Fennica's tasks is deploying the capping stack designed to stop an oil spill.

Its critical role in the program raises questions about whether Shell will need to adjust its drilling plans, and face a delay as it attempts to resume drilling that was suspended for three years after the grounding of the drill rig Kulluk on an island near Kodiak.

Shell said in a statement the Fennica's crew was alerted to a leak in the ballast tank on Friday shortly after leaving Dutch Harbor. After inspection, a "small breach" in the hull was found. The cause is unknown.

"All appropriate authorities were promptly notified and repair options are being considered," said the statement emailed by Luke Miller, a spokesperson with Shell.

"At this point we do not anticipate any impact on the season but it's too early to know for sure. Any impact to our season will ultimately depend on the extent of the repairs," the statement said.

Miller said the breach is about 39 inches long and about 2 inches wide.

"This is obviously an issue we take very seriously," said the statement. "The Fennica was traveling in charted waters, significantly deeper than her draft, with a qualified harbor pilot. Whatever the cause of this breach, the marine industry in Dutch Harbor will benefit from learning how it happened."

Unalaska Mayor Shirley Marquardt said some marine pilots in Dutch Harbor suspect the ship may have been damaged by underwater debris, possibly left over from World War II -- a common threat in the rugged region that might not show up on charts until ships are gouged or massive anchors haul up waste.

Once, a concrete mixing truck was pulled up, she said. Pilots have said massive anchors abandoned on the sea floor with flukes and shanks several feet long could have caused the Fennica's gashing, she said.

She said the crew of the National Oceanic and Atmospheric Administration vessel Fairweather, a hydrographic survey ship in Dutch Harbor as part of an effort to update nautical charts in the region, has agreed to survey the area where the damage may have occurred. That work is supposed to begin Wednesday, she said.

"Pilots are going, 'What the heck is down there?' " she said.

Pilots have told her the damage is a thin slice "like you were opening up a can," she said.

"There is nothing on the charts (on the route)," Marquardt said. "It surprised everyone. They thought it was the anchor getting caught on something."

Rick Entenmann, president of the Alaska Marine Pilots for the Western Alaska region based in Dutch Harbor, said the U.S. Coast Guard is investigating the incident.

He said seasoned pilot Rick Murphy was piloting the Fennica when it was damaged. Murphy was traveling Tuesday and not available for comment.

Entenmann said the damage occurred as the ship was transiting past the east side of Hog Island. The area is commonly traveled, but a few feet this way or that can make all the difference. He said he's certain the pilot won't be found at fault.

"He should have had 10 or 11 feet under his keel," he said. "This is one of those things. It's not on the chart."

"Who knows what's down there," Entenmann said. "It could be something from some vessel back in day, but it's got to be a pretty good-sized thing to be sticking up 9 to 11 feet above the bottom. We're just curious as hell to see what it is."

Marquardt said it's unfortunate Shell might be blamed for the accident by critics who don't support drilling in the U.S. Arctic Ocean, though the company was following proper procedures with a longtime marine pilot handling the vessel.

"So many things like this happen in the marine industry in Dutch Harbor and people just go, 'Oh they were lucky.' But when it's Shell, people who have no marine experience whatsoever or have never been to Dutch Harbor say, 'See they don't know what they were doing.'"

After meeting numerous regulatory requirements in an effort that began a decade or so ago, Shell is awaiting two permits -- authority for drilling two wells -- before it begins to punch holes this summer.

But aside from the damage to the Fennica, there are questions about whether Shell will be delayed in its plans to simultaneously drill two wells in the Chukchi Sea.

A U.S. Fish and Wildlife Service letter of authorization allowing incidental take of certain marine mammals specifies that simultaneous drilling can occur only if wells are 15 miles apart.

Shell's proposed plan calls for those first two wells to be spaced 9 miles apart, posing questions about how the company will proceed.

The statement sent by Miller said Shell "still intends to accomplish meaningful work in the weeks ahead. That includes drilling in the Chukchi Sea."